Add ability to specify alternative text for media attachments (#5123)

* Fix #117 - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference

app/javascript/mastodon/features/ui/components/media_modal.js

@@ -76,9 +76,9 @@ export default class MediaModal extends ImmutablePureComponent {
       const height = image.getIn(['meta', 'original', 'height']) || null;
 
       if (image.get('type') === 'image') {
-        return <ImageLoader previewSrc={image.get('preview_url')} src={image.get('url')} width={width} height={height} key={image.get('preview_url')} />;
+        return <ImageLoader previewSrc={image.get('preview_url')} src={image.get('url')} width={width} height={height} alt={image.get('description')} key={image.get('preview_url')} />;
       } else if (image.get('type') === 'gifv') {
-        return <ExtendedVideoPlayer src={image.get('url')} muted controls={false} width={width} height={height} key={image.get('preview_url')} />;
+        return <ExtendedVideoPlayer src={image.get('url')} muted controls={false} width={width} height={height} key={image.get('preview_url')} alt={image.get('description')} />;
       }
 
       return null;
@@ -90,6 +90,7 @@ export default class MediaModal extends ImmutablePureComponent {
 
         <div className='media-modal__content'>
           <IconButton className='media-modal__close' title={intl.formatMessage(messages.close)} icon='times' onClick={onClose} size={16} />
+
           <ReactSwipeableViews onChangeIndex={this.handleSwipe} index={index} animateHeight>
             {content}
           </ReactSwipeableViews>

app/javascript/mastodon/features/ui/components/video_modal.js

@@ -23,6 +23,7 @@ export default class VideoModal extends ImmutablePureComponent {
             src={media.get('url')}
             startTime={time}
             onCloseVideo={onClose}
+            description={media.get('description')}
           />
         </div>
       </div>

app/javascript/mastodon/features/ui/util/async-components.js

@@ -90,10 +90,6 @@ export function MediaGallery () {
   return import(/* webpackChunkName: "status/media_gallery" */'../../../components/media_gallery');
 }
 
-export function VideoPlayer () {
-  return import(/* webpackChunkName: "status/video_player" */'../../../components/video_player');
-}
-
 export function Video () {
   return import(/* webpackChunkName: "features/video" */'../../video');
 }